Costs are rising sharply - is additional financing currently possible?

how far along is the construction?

Read the thread?
There isn't even a clear cost calculation yet, just a "we have started." With whatever.

But I don't see it as critically, or rather I know some who have lived for years in a half-finished house, or whose outdoor facilities were not finished even after 10 years.

Well, we're talking about several hundred thousand euros. Half-finished is really half-finished. Definitely far from "you can live in it." And I certainly don’t want to have the discussion with the bank about having a shell construction for years...

I assume the thread will die down now anyway, since the OP is no longer responding.

Well, the OP wrote that they have started with the construction. What exactly that means, they have remained silent about despite several inquiries. But honestly, I also don't think anything more will come of it.

Well, we're talking about several hundred thousand euros. Half-finished really means half-finished. It's a long way from "you can live in there." And I don't want to have the discussion with the bank about having a shell for years either...

an unfinished attic or basement saves a few hundred thousand euros

Hm. I had asked what professional background is present. If the two are now, for example, a trained machinist and a trained plasterer, and have electricians, painters, installers in their circle of friends, then there is actually still hope.

But even there it gets tight, very, very tight.
My little house, turnkey, now costs including ancillary building costs for 152m² 350,000 EUR.
At Town & Country in Saxony-Anhalt. You won’t get it cheaper anywhere.
Now calculate that up to 240m², basement, garage... you quickly come to over 700,000 EUR. That is already more than the budget available.

Save on own work, but only if you do it promptly, because that delays moving in. Electric, sanitary, plaster, screed, that has to be finished, otherwise you get worn out. Doing that yourself, the construction can quickly take 2, 3, 4 years. Especially since as a private person you currently can’t even get the materials or only at completely overpriced prices.

What’s left then? Laying floors and painting walls.

an unfinished attic or basement saves a few hundred thousand euros.

Not just any attic, but the upper floor as a full story. That will probably also include the bedroom and a proper bathroom. You need that for living.
And the basement is a given. Otherwise, a complete redesign is necessary because the heating system takes up a lot of space that is not available on the ground floor. So, a new floor plan + new building permit.

Trying to save quickly and not finish building is difficult — assuming construction has actually started.

And finally, the problem with the bank. If the house ultimately stands only as a refined shell, I would quickly refer you as a customer to the renovation department, because the value of the property has not been reached. Loan termination or additional collateral demands, etc., would be knocking loudly at the door in such a situation.

an unfinished attic or basement saves a few 100,000.--€

It would be nice if you also read the threads you answer sometimes. At least the information the OP provides. Otherwise, it could just lead to a pointless post that lacks any added value. That's also why people sometimes get banned when the limit is exceeded.